The present invention is related to a radio communication system and more particularly is related to a radio communication system capable of automatically evaluating carrier frequencies for transmission quality and selecting the frequency with the highest quality for operation.
Broadcast communication involving mobile radios is more difficult than fixed radio communication. Along with the normal broadcast communication problems of propagation and interference, the mobile radio has the additional problems of low power limitation, antenna coupling losses, poor antenna patterns, and changing antenna losses and patterns. Full time mobile radio communication requires the availability of several carrier frequency channels spread across the broadcast spectrum of interest to accommodate short-term changes in ionospheric sky-wave propagation. Fixed radio communication circuits can take advantage of multiple frequency assignments and switch to frequencies which are propagating well and have low interference. A method of frequency selection is taught in U.S. Pat. No. 3,617,891, issued to D. H. Covill, entitled OPTIMUM FREQUENCY DETERMINING RADIO COMMUNICATION SYSTEM, filed May 26, 1969.
The Covill patent discloses a structured frequency selection method suitable for fixed radio circuits. The method comprises a synchronous search mode which is entered first and an asynchronous search mode, entered only upon the failure to qualify a frequency channel during the synchronous search. The asynchronous search mode entails transmission of a test signal by a first radio on each frequency in a set of frequencies taken at a slow rate. The receiver of a second radio is tuned at a higher rate to each in the set of frequencies for detection of the test signal on the frequency of transmission. The asynchronous mode requires a degree of synchronism in that the second radio, upon detecting the test signal, must immediately retransmit it at the same frequency. The first radio must then detect the test signal, evaluate the signal-to-noise ratio at the carrier frequency, and transmit an answer signal to the second radio if the transmission quality of the frequency channel is acceptable. This method does not qualify every available carrier frequency channel in the set of assigned frequencies because it is interrupted when the first channel is qualified. Hence, the freqency channel with the highest transmission quality is not necessarily selected. Furthermore, the multiplicity of transmissions required limits the reliability of the method and extends the time required to evaluate the whole set of frequencies.
Operators of battery operated mobile radios on the move in the field presently require a rapid, reliable means of determining the optimum frequency for use at a given time over a given path. An asynchronous selection method is preferred because the time required for antenna tuning before transmission is uncontrolled; it varies according to the constantly changing location of a mobile radio. The method must involve as few transmissions as possible in order to preserve battery power. Minimization of transmission is also important when the security of the circuit is at stake. Finally, a method is desired which will evaluate all available carrier frequencies for the purpose of selecting the best among them.